Pesticidal treatment of stored goods, enclosures, structures and works of art, with sulphur compounds

ABSTRACT

The present invention is directed to the pesticidal treatment of stored foodstuffs, chambers, structures and works of art, using a volatile sulphur compound of general formula:  
                 
in which R represents an alkyl or alkenyl radical containing from 1 to 4 carbon atoms, n is equal to 0, 1 or 2, x is a number ranging from 0 to 4, and R′ represents an alkyl or alkenyl radical containing from 1 to 4 carbon atoms or, only if n=x=0, a hydrogen atom. These sulphur compounds (in particular dimethyl disulphide) are applied by nebulization directly to the material to be treated.

FIELD OF THE INVENTION

The present invention relates to the field of the pesticidal treatmentof stored foodstuffs, chambers and structures, and works of art, and itssubject is more particularly a pesticidal treatment by nebulizationusing sulphur compounds.

BACKGROUND OF THE INVENTION

Currently, the pesticidal treatment of stored foodstuffs, chambers,structures and works of art is essentially carried out according to twotechniques:

-   -   fumigation, using in particular methyl bromide (MB) and        phosphine (PH₃) which act in the gaseous state on the        respiratory chain of the target organisms,    -   so-called “contact” treatments using, for example,        organophosphorus insecticides, in particular dichlorvos, which        act in the condensed state directly on the target organisms.

Methyl bromide (MB) exhibits in the gaseous state excellent nematicidal,fungicidal, insecticidal and bactericidal properties. Unfortunately,this compound contributes to depletion of the ozone layer and, inaccordance with the Montreal Accord (1992), by 2005 it should no longerbe used in industrialized countries. Phosphine (PH₃) exhibits majordisadvantages such as its toxicity, the duration of the treatments andthe corrosion of equipment in which this compound is used (Pest Control(1999) Vol. 67(1), p. 46).

Moreover, organophosphorus compounds have an activity spectrum reducedto their insecticidal action and generate toxic residues. In addition,since they only act through contact directly with the target organisms,they cannot reach the so-called hidden forms of these organisms;accordingly, the eggs and larvae of the insects present inside thegrains of wheat and maize are not killed by the insecticide. This typeof treatment is markedly less effective.

There is therefore an urgent need to provide users with alternativesolutions which are effective and which are as environmentally friendlyas possible. Examples of fumigant products under development for theseapplications are known and are described in various articles, inparticular in World Grain, February 2001, p. 28-30 and Crop Prot. (2000)19 (8-10), p. 577-582); but the solutions envisaged also exhibit majordisadvantages such as their high cost (methyl iodide), their lowavailability (carbon oxysulphide) or their toxicity (sulphurylfluoride).

Another family of compounds, the sulphur-containing products derivedfrom substances produced by certain plants, for example the Alliums, isalso known for its pesticidal and repellent activity and has alreadybeen the subject of numerous publications (Ecologie (1994) 25(2), p93-101, Ed. Tec. and Doc., Biopesticides d'origine végétale (2002) p77-95, Insect Sci. Applic. (1989) 10(1), p. 49-54, Pestic. Sci. (1999),Vol. 55, p. 197-218). The use of these sulphur compounds as fumigants inthe treatment of stored foodstuffs has never been generally accepted.Patent application FR-A-2 779 615 filed in 1997 discloses a particulartechnique for the aplication of such treatments: the circulation of gasin a loop in a sealed silo by means of a pump. Indeed, it is essentialfor a fumigant for the treatment of stored foodstuffs to diffuse thetreatment rapidly in the gaseous form in the mass to be treated.However, it has been demonstrated by the applicant that thesesulphur-containing products spontaneously diffuse too slowly in a massof grains, causing large differences in the gas concentration insidethis mass and therefore an inefficiency in the treatment. The solutionproposed in the abovementioned patent application, consisting incarrying the active gas in a stream of air or of air enriched with CO₂,is an expensive technique which is not available on the majority ofexisting installations and which is difficult to apply because of thelack of leaktightness in the majority of installations for the storageof foodstuffs.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a graph of dimethyl disulphide concentration versus time in atreated chamber.

DETAILED DESCRIPTION OF THE INVENTION

It has now been found that the sulphur compounds of general formula:

in which R represents an alkyl or alkenyl radical containing from 1 to 4carbon atoms, n is equal to 0, 1 or 2, x is a number ranging from 0 to4, and R′ represents an alkyl or alkenyl radical containing from 1 to 4carbon atoms or, only if n=x=0, a hydrogen atom, are particularlyadvantageous fumigants for the treatment of stored foodstuffs when theyare applied by nebulization like contact insecticides. The subject ofthe present invention is therefore a pesticidal treatment of storedfoodstuffs, characterized in that at least one volatile sulphur compoundof formula (I) is applied by nebulization. The sulphur compounds offormula (I) are all sufficiently volatile to pass rapidly, under theusual temperature and pressure conditions, from the liquid state to thegaseous state in which they are active. The nebulization technique hasthe advantage of applying the sulphur compounds directly at the sitewhere they have to act, which solves the problem of their too slowspontaneous diffusion in the mass to be treated. Furthermore, it can beused in installations with imperfect leaktightness and in existingequipment intended for the application of contact insecticides.

The sulphur compounds of formula (I) not only exhibit pesticidalproperties towards insects, fungi, bacteria, viruses, nematodes,arachnids and rodents, but also repellent properties towards insects,arachnids and rodents.

They are therefore perfectly suitable for the pesticidal and/orrepellent treatment of dry or moist stored foodstuffs, such as wheat,maize, rice, apples, dry fruits and dry processed products such as forexample animal foods, chambers and structures such as silos, timber,wooden structures including those used for the culture of mushrooms,including quarantine treatments. The invention also relates to thecompounds of formula (I) applied by nebulization for the pesticidaland/or repellent treatment of works of art and other valuable objectsderived from plant or animal products, such as for example paintings,sculptures and fabrics.

As substitutes for methyl bromide, the compounds of formula (I) are allthe more advantageous since they are already present in nature, beingderived from the natural degradation of crucifers and alliums. Inparticular, the thiosulphinates, included in general formula (I), areproducts which are naturally emitted when alliums are ground and, inthis regard, can be used in agricultural biology for the treatment ofstored foodstuffs. Moreover, given that they do not contain halogenatoms which generate halogenated radicals responsible for the catalyticdestruction of the stratospheric ozone, the compounds of formula (I) arewithout danger for the ozone layer.

As nonlimiting examples of radicals R and R′, there may be mentionedmethyl, propyl, allyl and 1-propenyl radicals. Among the compounds offormula (I), the compounds for which n=0 are preferred. Other preferredcompounds are the disulphides (n=0, x=1) and more particularly dimethyldisulphide (DMDS).

The compounds of formula (I) can be used in the pure state or in variousforms which, depending on the nature of the compound (I), can be anaqueous emulsion, a microemulsion, a microencapsulated product, asolution in water or in an organic solvent. All these formulations canbe made according to methods well known to a person skilled in the art.

The organic solvents which may be used to dissolve the compounds offormula (I) according to the invention are hydrocarbons, alcohols,ethers, ketones, esters, halogenated solvents, mineral oils, naturaloils and their derivatives, and aprotic polar solvents such asdimethylformamide, dimethyl sulphoxide or N-methylpyrrolidone.Biodegradable solvents, more particularly methyl esters of rapeseed andsoybean oils, are particularly suitable.

The compounds of formula (I) are applied, in the forms described aboveaccording to the method known to a person skilled in the art for contactinsecticides, by nebulization of the pure or formulated product directlyonto the material to be treated, in an atmosphere of pure air or of airenriched with CO₂.

The doses of compounds (I) to be used in order to obtain the desiredeffect should meet an objective in terms of CT, that is to say theproduct of the concentration C of active substance in the air and of thetime T during which the product is allowed to act. The product CTindicates the cumulative dose of the active compound to which thepathogenic organisms are subjected during the treatment. The objectiveis to reach a value of the product CT, called lethal CT, correspondingto the complete destruction of the target organisms. For an optimumtreatment, the value of the lethal product CT should be reached asrapidly as possible and in the most homogeneous manner possible in themass or on the material to be treated; this is obtained using the methodof application of the compound by nebulization.

The lethal CT values are generally between 20 and 200 ghm⁻³ and dependon the nature of the compound (I), the level of infestation, the natureof the target organism, the type of material to be treated, and theventilation of the chambers.

Once the lethal effect has been obtained, the product (I) is removed byventilation and therefore generates no residue on the material to betreated.

The following examples illustrate the invention without limiting it.

EXAMPLE 1 (COMPARATIVE)

In this example, dimethyl disulphide (DMDS) is used as compound offormula (I) and its spontaneous diffusion in a silo of wheat grains isstudied.

Materials and Methods:

Temperature: 20° C.

Dimensions of the silo:

-   -   total height: 80 cm    -   diameter: 46 cm    -   total volume: 1301    -   filling level: 80%, that is 79 kg of wheat    -   grain height: 72 cm

Pure DMDS is deposited at a concentration of 30 gm⁻³ at the surface ofthe grains using a syringe.

There are then measured by gas chromatography as a function of time (inhours) the concentrations of DMDS in gaseous form at the surface of thesilo of wheat grains where DMDS is introduced (point A: 0 cm), and at 36cm below the level of the grains in the silo (point B: −36 cm) and atthe lower end of the silo (point C: −72 cm).

The products CT of the concentrations C of DMDS by the time ofmeasurement T are determined at the various points of measurement over aperiod of 3 days. The CT values in ghm⁻³ indicated in Table 1 are thusobserved. TABLE 1 CT Time (days) A B C 1 542 46 4 2 630 80 14 3 821 18062

The products CT for DMDS are not homogeneous in the entire thickness ofthe silo, even after a long residence time. The conditions for theapplication of DMDS in this example do not make it possible to obtain asufficiently rapid spontaneous diffusion of the fumigant.

EXAMPLE 2

This example, which does not use application by nebulization, makes itpossible to obtain a concentration gradient over the height of thechamber and to thus determine the values of the lethal product CT in asilo of wheat grains infested with two pathogenic organisms usuallyencountered as principal pests of stored foodstuffs. These two organismsare Sitophilus granarius and Sitophilus orizae.

Materials and Methods

Temperature: 20° C.

Dimensions of the silo:

-   -   total height: 40 cm    -   total volume: 3.3 l    -   filling level: 75%, that is a grain height of about 33 cm.

99 mg of pure DMDS are deposited at the surface of the grains using asyringe. The samples collected consist of bundles of 50 g of wheatcontaining all the stages of development of the pathogenic organisms.After degassing by aeration of the silo at the times T, the bundles ofwheat are screened and the adults are counted immediately afterscreening (D+0) and after 14 days (D+14). The wheat containing the otherstages of development is then examined for 5 weeks.

The products CT of the concentrations C of DMDS by the times ofmeasurement T are determined and the efficacy of the treatment inrelation to the 2 pathogenic organisms studied for all the stages ofdevelopment is monitored as a function of these products CT. The resultsare expressed:

-   -   as a percentage of the adult mortality at the times (D+0) and        (D+14).    -   as a percentage of net emergence reduction (% NER) relative to        an untreated control batch for the different stages of        development of these organisms (that is to say eggs and 2nd        generation, eggs and 1st stage larvae, 1st and 2nd stage larvae,        2nd and 3rd stage larvae, 3rd and 4th stage larvae, 4th stage        larvae and nymphs), which corresponds to the following formula:        % NER=(number of insects which emerged live from the control        batch−number of insects which emerged live from the treated        batch)/(number of insects which emerged live from the control        batch).

A—Case of Sitophilus Granarius

The results are assembled in Table 2. TABLE 2 % NER for the variousstages of the batch treated with DMDS Eggs 1st and IInd and IIIrd andIVth Total % adult Eggs and and 1st IInd IIIrd IVth stage No. of —mortality 2nd stage stage stage stage larvae insects % CT D + 0 D + 14generation larvae larvae larvae larvae and nymphs emerged NER 5.8 0 8 1341 20 13 3 21 202 25 6.1 3 9 33 5 80 16 19 54 208 23 15.0 64 93 25 54 8871 26 75 126 54 26.9 92 92 20 79 88 94 65 92 75 72 50.5 100 100 93 100100 100 97 100 4 99 88.4 100 100 95 100 100 100 100 100 2 99 160.4 100100 100 100 100 100 100 100 0 100 Number of insects which emerged livefrom the untreated control batch 40 120 25 31 31 24 271

Table 2 shows that a product CT of 160 gh.m⁻³ results in a mortality of100% for all the stages of development of Sitophilus granarius and thatfrom CT values greater than 50 gh.m⁻³, 100% mortality is obtained forthe majority of the stages. The pesticidal efficacy of DMDS against thepathogenic organism Sitophilus granarius is evaluated in terms of theproduct CT at about 100 ghm⁻³ (useful CT).

B—Case of Sitophilus Orizae

Under the same trial conditions, this time on organisms of theSitophilus orizae type, similar results are obtained. The pesticidalefficacy of DMDS against the pathogenic organism Sitophilus orizae isevaluated in terms of the product CT at about 100 ghm⁻³ (useful CT).

EXAMPLE 3

In this example, use is made of DMDS formulated as a 30% solution in amethyl ester of rapeseed oil (Radia®7961 provided by the OLEON company).Its efficacy is studied in a silo of wheat grains by monitoring itsabsorption onto the grains as a function of time after its applicationby nebulization.

Materials and Methods

Temperature: 20° C.

Chamber:

-   -   total volume: 31    -   total height: 28 cm    -   filling with 1 kg of wheat grains, that is a height of 15 cm

There are introduced 90 mg of pure DMDS as a solution in 210 mg ofRadia®7961 by 2 successive sprayings by means of a nebulizer. Thechamber is immediately mechanically stirred so as to rapidly homogenizethe gas concentration in the treated mass.

The DMDS concentrations in the atmosphere in the chamber are monitoredfor 5 days by SPME-GC-MS analysis. FIG. 1 illustrates the concentrationof DMDS in the atmosphere of the chamber as a function of time for thefirst 30 hours. It is observed that the concentration of DMDS in theatmosphere of the chamber decreases regularly during the first 5 hoursto reach the value of 3 gm⁻³ which then remains constant during the 5days. The useful product CT of 100 ghm⁻³ corresponding to an optimumefficacy of DMDS is reached after 30 hours of treatment under theapplication conditions of the invention.

1. Pesticidal treatment of stored foodstuffs, chambers, structures andworks of art, comprising appling by nebulization at least one volatilesulphur compound of general formula:

in which R represents an alkyl or alkenyl radical containing from 1 to 4carbon atoms, n is equal to 0, 1 or 2, x is a number ranging from 0 to4, and R′ represents an alkyl or alkenyl radical containing from 1 to 4carbon atoms or, only if n=x=0, a hydrogen atom:
 2. Treatment accordingto claim 1, wherein R and R′ of formula (I) are selected from the groupconsisting of methyl, propyl, allyl and 1-propenyl radicals. 3.Treatment according to claim 1, wherein n is equal to zero.
 4. Treatmentaccording to claims 1, wherein said sulphur compound is a disulphide. 5.Treatment according to one of claims 1, wherein said sulphur compound isdimethyl disulphide.
 6. Treatment according to one of claims 1, whereinsaid sulphur compound is applied by nebulization in the form of anaqueous emulsion, a microemulsion or a solution in a biodegradablesolvent.
 7. Treatment according to one of claims 1, wherein the productCT of the concentration of sulphur compound applied (C) and of the time(T) for which the sulphur compound is allowed to act, in order to obtaincomplete destruction of the target organisms, is between 20 and 200ghm⁻³.